1.Field of the Invention
The present invention relates to a semiconductor integrated circuit device (IC) for driving a liquid crystal display (referred to as LCD hereinafter) panel, and more particularly to a semiconductor IC adapted for driving an active matrix type LCD panel for displaying image signals such as television signals.
2. Description of the Prior Art
The use of the LCD panel started with the electronic disk computer and wrist watch, and as of now it is spreading to the color television receiver, word processor and the monitor equipment of various kinds of OA apparatus and various kinds of computer terminal. This advancement is being accelerated by the progress of the fabrication technology of the active matrix type LCD panel, particularly of the active matrix type LCD panel that makes use of the thin film transistors (referred to as TFTs hereinafter).
An LCD panel has a pair of transparent glass plates that are arranged mutually parallel maintaining an extremely narrow gap, and a liquid crystal material that is interposed in the gap. On the inner surface of one of these transparent glass plates are arranged in matrix form a large number of pixel forming electrodes (pixel electrodes), and on the inner surface of the other glass plate are arranged counter electrodes that oppose respectively to these pixed electrodes. The electrode pairs that consist of respective. ones of the pixel electrodes and respective ones of the counter electrodes form liquid crystal cells together with the liquid crystal material that is interposed between them, and desired image display is achieved by selectively controlling the light transmission characteristic of the liquid crystal cells through application of voltages to these electrode pairs.
In a TFT-base LCD which is the representative of the active matrix type LCD, there are formed a large number of row wiring members which extend mutually parallel along the row direction between the rows of the pixel electrodes of the matrix array and a large number of mutually parallel column wiring members formed between the columns along the column direction perpendicular to the row wiring members. At each of the noncontact intersections of these row wiring members and the column wiring members there is arranged a switching TFT with its gate electrode connected to the row wiring member, its source electrode connected to the column wiring member, and its drain electrode connected to the image electrode. Reflecting the connective relation to each electrode of the TFT, a row wiring member and a column wiring member will be referred to hereinafter as a gate driving line and a source driving line, respectively. The representative LCD panel for color television or personal computer monitor currently on the market has 400 gate driving lines and 640 source driving lines. A gate driving pulse train corresponding to the vertical sweep voltage in a CRT display device is supplied stepwise from a gate drive circuit to these gate driving lines. In other words, a line of large number of liquid crystal cells connected to each of these gate driving lines represents an image for one line component of horizontal scanning of the image signal.
On the other hand, the source driving lines receive, synchronized with horizontal shift pulses of horizontal scanning, from a source drive circuit the supply of a train of sample values each representing the sample value of the image signal voltage to be displayed. The source drive circuit is equipped with a shift register which has register stages equal in number to the source driving lines of the LCD, and generates in succession the horizontal shift pulse synchronized with the horizontal synchronizing pulse of the image signal, and a sample/hold circuit which has sampling circuits equal in number to the register stages for sampling an analog image signal to be displayed, in response to the horizontal shift pulse, and holds the respective sampled values.
Since it is necessary to highly integrate the gate drive circuit and the source drive circuit corresponding to the high density arrangement of the gate driving lines and the source driving lines on the LCD panel, they are put on the market each in the form of a semiconductor IC. As for the LCD for color television signal display a panel drive circuit is normally formed by combining a gate driving IC and a source driving IC.
The plurality of sample/hold circuits constitutes a buffer for applying the sampled values of the amplitude the input image signal to liquid crystal cells having a large time constant, and carries out the polarity inversion of the applied signal in order to prevent the deterioration of liquid crystal cells characteristic of the LCD panel. Namely, as is disclosed in the specification of U.S. Pat. No. 3,653,745, if driving voltages of the same polarity are applied continuously to a liquid cell, electrochemical changes are generated in the pixel electrode and the counter electrode, deteriorating the sensitivity of display and the luminance. In order to prevent this it is necessary to constantly invert the polarity of the voltage applied to the liquid crystal cell. As disclosed in the specification of U.S. Pat. No. 4,842,371, in a TFT-base active matrix LCD the polarity inversion is carried out for each line of horizontal scanning, and in the combination of interlace scanning for suppressing the flickering of the screen. That is, for odd-numbered fields, image signal is supplied only to odd-numbered lines, namely, the first, third, fifth, seventh lines, and so on from the top end among the gate driving lines, and for even-numbered fields, image signal is applied only to even-numbered lines in the order of the second, fourth, sixth, eighth lines and so on. Therefore, in conformity with this regularity, liquid crystal cells belonging to respective gate driving lines undergo polarity inversion for every line of horizontal scanning.
In the LCD drive circuit disclosed in the specification of U.S. Pat. No. 4,842,371 the polarity inversion is achieved by controlling the applied voltages to the counter electrodes, so that the LCD panel drive circuit is complicated in proportion to the number of voltages for the control objects and the required driving power is accordingly high. In contrast, in the semiconductor IC for LCD driving, announced in a preliminary data sheet entitled "MOS Integrated Circuit uPD16400" published in March 1990 by NEC Corporation which is the assignee of this invention, a configuration which commonly connects all of the counter electrodes to a panel reference potential point is adopted, and hence the drive circuit is simplified and the power consumption is reduced accordingly. However, it is constructed such that the potential of the counter electrodes is fixed to that at the panel reference potential point and only the applied voltages to the pixel electrodes are controlled over both positive and negative polarities. Therefore, it is required to extend the range of the output voltage of an output stage transistor of the sample/hold circuit in the panel driving IC. This point will be described in detail in the following:
Each of a plurality (same number as the source driving lines) of sample/hold circuits included in the source drive circuit comprises a first circuit part and a second circuit part which alternately carry out the extraction and the holding of voltage sample values of the image signal of each horizontal scanning line in one field and the outputting of the held voltage sample value, where these two circuit parts commonly receive the supply of the image signal and a polarity display pulse of the voltage applied to the LCD cell (polarity display pulse) that is synchronized with the horizontal synchronizing signal of the image signal. The first circuit part includes an AND circuit which generates the logical product output of the horizontal shift pulse and the polarity display pulse, a first sampling switch which samples the image signal voltage in response to the logical product output, a first capacitor which holds the sample value that is the output of the switch, a first buffer amplifier which amplifies the output of the capacitor, and a first output switch which leads the output of the amplifier to the corresponding source driving line. Analogously, the second circuit part includes a NAND circuit which operates complementarily to each of the AND circuit, the first sampling switch, and the output switch, a second sampling switch, and a second output switch, where it is constructed such that the output of the second sampling switch is led, after being hled in a second capacitor, to the source driving line through a second buffer amplifier and the second output switch. In one field period, the first sampling switch is closed for every odd-numbered horizontal scanning line, for example, and the sample value of the image signal voltage is held in the first capacitor. During this period the first output switch stays in the open state, and no driving voltage output is generated from the first circuit part to the source driving line. In contrast, the second output switch of the second circuit part remains closed during this period so that the sampled voltage value held in the second capacitor during the one line in the preceding period is output to the corresponding source driving line through the second buffer amplifier and the second switch. The amplitude of the image signal undergoes polarity inversion for every line in order to prevent the deterioration of the liquid crystal cells, and moreover, the presence or absence of polarity inversion for every line is reversed for every field. By so doing, liquid crystal cells belonging to a certain line which is subjected to the application of a positive polarity image signal during one field period, for example, are arranged to receive without fail the application of a negative polarity image signal during the next field period.
However, in the conventional LCD drive circuit of the above-mentioned type, although the reversion of the presence or absence of polarity inversion of the image signal for every line is implemented for every field or for every frame, the corresponding reversion of the presence or absence of the polarity inversion is not implemented for the polarity display signal of one line length of the horizontal scanning extracted synchronized with the horizontal synchronizing signal of the image signal. In other words, the presence or absence of polarity inversion for one line portion of the image signal and the presence or absence of polarity inversion which is displayed by the polarity display signal could be different. Consequently, the first circuit part of the sample/hold circuit corresponding to a certain pixel electrode of a certain horizontal scanning line, is required not only to carry out the holding and the buffer amplification of the positive sample value upon receipt of an image signal lacking polarity inversion during one field (or frame) period, but also to carry out the holding and the buffer amplification of negative sample value of an image signal upon receipt of supply of image signal with polarity inversion during the next field (or frame) period. An entirely analogous requirement applies also to the second circuit part. Namely, the capacitor for holding the sample value holds sample values of both positive and negative polarities, and correspondingly the range of voltage value that has to be handled is expanded, which requires that the breakdown voltage Of the transistor (FET) of the buffer amplifier has to be raised accordingly.
The process of forming the FET of the buffer amplifier within the LCD driving semiconductor IC includes, when the FET is required to have a high breakdown voltage, two times of diffusion process in order to form a double structure consisting of a heavily doped region and a lightly doped region for each of the source and the drain regions. The dimensional accuracy of the impurity diffused region by these diffusion processes dominates the accuracy of the output voltage of the buffer amplifier. Because of this, the accuracy of the source driving voltage output of the presently commercially available semiconductor IC for LCD driving is about 150 mV at the most. On the other hand, the gradations of images that can be displayed by liquid crystal is improved to about 256 thanks to the improvement of the LCD panel, the CPU of the personal computer, and the like, so it is necessary to enhance the accuracy of the output voltage of the semiconductor IC for driving. This is because the fidelity of the gradation for every pixel of an image to be displayed cannot be ensured for a driving TC which does not satisfy this requirement.